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From: Vince Weaver <vincent.weaver@maine.edu>
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Date: Wed, 28 Jun 2017 08:40:30 -0400 (EDT)
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To: Mark Rutland <mark.rutland@arm.com>
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Subject: Re: [PATCH] perf/core: generate overflow signal when samples are
 dropped (WAS: Re: [REGRESSION] perf/core: PMU interrupts dropped if we
 entered the kernel in the "skid" region)
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On Wed, 28 Jun 2017, Mark Rutland wrote:

> On Wed, Jun 28, 2017 at 11:12:48AM +0100, Mark Rutland wrote:

> Instead of bailing out early in perf_event_overflow, we can bail prior
> to performing the actual sampling in __perf_event_output(). This avoids
> the information leak, but preserves the generation of the signal.
> 
> Since we don't place any sample data into the ring buffer, the signal is
> arguably spurious. However, a userspace ringbuffer consumer can already
> consume data prior to taking the associated signals, and therefore must
> handle spurious signals to operate correctly. Thus, this signal
> shouldn't be harmful.

this could still break some of my perf_event validation tests.

Ones that set up a sampling event for every 1M instructions, run for 100M 
instructions, and expect there to be 100 samples received.

If we're so worried about info leakage, can't we just zero-out the problem 
address (or randomize the kernel address) rather than just pretending the 
interrupt didn't happen?

Vince